Heat treat configuration for porous carbon-carbon composites

ABSTRACT

A method of heat treating a substrate for a fuel cell includes stacking substrates to form a group. A dimension is determined for a plate corresponding to a resulting mass that is less than a predetermined mass. The plate is arranged above the group to apply a weight of the plate to the group. The resulting masses for spacer plates and intermediate lifting plates, for example, are minimized to reduce the pressure differential between the bottom and top substrates in the heat treat assembly. In another disclosed method, a dimension for a plate, such as a top plate, is determined that corresponds to a resulting mass that is greater than a predetermined mass. The plate is arranged above the group to apply a weight of the plate to the group. The top plate resulting mass is selected to minimize a variation in the average pressure of the substrates throughout the heat treat assembly.

BACKGROUND

This disclosure relates to a porous carbon-carbon composite suitable foruse as a substrate in fuel cells, for example.

Some types of fuel cells, such as proton exchange membrane andphosphoric acid fuel cells (PEMFC and PAFC), use porous carbon-carboncomposites as electrode substrates, which are also referred to as gasdiffusion layers. One example fuel cell substrate and manufacturingprocess is shown in U.S. Pat. No. 4,851,304.

One typical method of making a substrate includes: (1) forming anon-woven felt from a chopped carbon fiber and a temporary binder by awet-lay paper making process, (2) impregnating or pre-pregging the feltwith a phenolic resin dissolved in a solvent followed by solvent removalwithout curing the resin, (3) pressing one or more layers of felt to acontrolled thickness at a temperature sufficient to cure the resin, (4)heat treating the felt in an inert atmosphere to between 750-1000° C. toconvert the phenolic resin to carbon, and (5) heat treating the felt inan inert atmosphere to between 2000-3000° C. to improve thermal andelectrical conductivities and to improve corrosion resistance. The artas illustrated by U.S. Pat. No. 4,851,304 is incomplete because it doesnot teach how to produce substrates with a uniform porosity, bulkdensity, and thickness in a high volume heat treating operation.

The porous carbon-carbon composites used in fuel cells typically have aporosity of 70-75%, which corresponds to a bulk density of 0.48-0.58g/mL for an example substrate. It is desirable to control the porositywithin a tight range because it affects the properties of the substratethat, in turn, influence the performance of the fuel cell. The thicknessof these substrates ranges from 0.12-2.00 mm, but thicknesses in therange of 0.12-0.50 mm are more typical. These substrates typically havea planform size of 50-100 cm×50-100 cm. The 2000-3000° C. heat treatingstep, frequently referred to as graphitization, is done in knowninduction or Acheson type furnaces in an inert atmosphere. A typicalfurnace load may contain a stack of approximately 2000 substrates and isabout 72-120 inches (183-305 cm) tall.

The thickness of each substrate decreases by about 33% during heat treatdue to pyrolysis of the thermoset resin. There is a tendency for thesubstrates to warp as a result of this shrinkage. Spacer plates areplaced between groups of 50-200 substrates in the heat treat stack tomaintain the flatness of the substrates as they shrink during heattreat.

Example prior art heat treat assemblies 11 are shown in FIGS. 1 and 2.The arrangement illustrated in FIG. 1 depicts a heat treat assembly of afirst generation substrate having a planform dimension D3. As the fuelcell was redesigned, a smaller substrate having a planform d3 wasdeveloped. However, the reusable tooling employed in the heat treatassembly 11 has not been changed as the substrates became smaller sincethere was no apparent need and due to the large expense of manufacturingnew tooling for the heat treat assemblies.

It has been found that the bulk density of the heat treated substratevaries with its position within the heat-treat stack and morespecifically with the local pressure within the heat treat stack. Oneskilled in the art can calculate the local pressure at any point in thestack by summing the weight of the substrates and tooling above thepoint and dividing it by the area of the substrates. FIG. 4 showssubstrate density versus position within the furnace for theconfigurations shown in FIGS. 2. The relevant tooling in this instanceconsisted of ½″×33″×33″ graphite spacer plates placed between each groupof 50 substrates. There was also a 48″ diameter lifting fixture and a33″×33′×4″ base plate in the center of the furnace load. The pressurevariation from the top to the bottom of this particular stack wasanalyzed and is shown in FIG. 5 as a graph of pressure versus positionin the furnace. The sharp discontinuity in the center is due to thelifting fixture and base plate. The over-all pressure range is small;but has a significant influence on the porosity and bulk density of thisporous carbon-carbon composite. The average pressure is 2.8 psi with arange of +/−2.3 psi or +/−82% from the top to the bottom. Substrates onthe bottom of the heat treat assembly are most dense and those on topare the least dense. This is particularly true of arrangements such asthose shown in FIG. 2. This results in a low process yield with asignificant number of parts being unacceptable because they do not meetthe density specification. There is a need for a heat treat toolingconfiguration that minimizes the pressure variation between the top andbottom of the heat-treat stack.

SUMMARY

A method of heat treating a porous carbon-carbon composite, such as asubstrate for a fuel cell, is disclosed. The methodology defines thecriteria for selecting the tooling configuration for heat-treating astack of porous carbon-carbon composites where the tooling is configuredsuch that the pressure variation between the top most part and thebottom most part in the heat-treat stack is less than +/−30%, andpreferably less than +/−15%.

The method includes stacking substrates to form a group. A plate isarranged above the group to apply a weight of the plate to the group.Multiple groups of substrates and spacers are then placed into a stack.The substrates per spacer, thickness of the spacer and planform of thespacer relative to the planform of the substrate, the height of thestack and the resulting masses for spacer plates and intermediatelifting plates, for example, are selected to minimize the pressuredifferential between the bottom and top substrates in the heat treatassembly.

In another disclosed method, a weight for a plate, such as a top plate,is determined. The plate is arranged above the group, in one exampleabove the topmost substrate in the heat treat assembly, to apply aweight of the plate to the group. The weight of the top plate isselected, in combination with the previously mentioned factors tocontrol the average pressure of the substrates, and hence the averagedensity of the substrate within the heat treat assembly.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of one example prior art heat treat assemblyusing a first generation substrate.

FIG. 2 is an example prior art heat treat assembly using a secondgeneration substrate that is smaller than the first generation substratewith the same heat treat assembly tooling as illustrated in FIG. 1.

FIG. 3 is a schematic view of one example heat treat assembly withreduced variation in the average pressure and decreased pressuredifferential between the top and bottom substrates in the heat treatassembly.

FIG. 4 is a plot of substrate density versus position within the heattreat stack.

FIG. 5 is a plot of substrate density versus pressure within the heattreat stack.

FIG. 6 is a plot of pressure versus position within the heat treat stackfor a configuration according to this disclosure having a lower pressuredifferential between the top and bottom substrates in the heat treatassembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Prior art heat treat assemblies 11 are shown in FIGS. 1 and 2. The heattreat assemblies 11 include carbon substrates and tooling. The toolingis used to move the substrates into and out of a furnace 12 and to applyweight to the substrates to prevent warping during heat treat. Thetooling is typically made of graphite but could be some other hightemperature material. FIG. 1 illustrates groups 18 an earlier substratedesign that measure 32 inches (D3)×32 inches×0.016 inches (81 cm×81cm×0.04 cm). The groups 118 include substrates of a later design thatmeasure 23 inches×23 inches×0.016 inches (58 cm×58 cm×0.04 cm).

Graphite spacer plates 20 of 33 inches (D2)×33 inches×½ inch (84 cm×84cm×1 cm) are placed between each group of fifty substrates, for example.The spacer plates 20 prevent the substrates from warping during heattreat. A 48 inches (122 cm) diameter, D1, lifting plate 14 and a 33inches×33 inches×4 inches (84 cm×84 cm×10 cm) base plate 16 are arrangedin the center of the furnace load. The term “diameter” represents thewidth of the object, and does not require the object to be circular.

The variation in density of the substrates versus position in the heattreat stack is shown in FIG. 4 for two different furnace runs. Thevariation in pressure versus position for the configuration in FIG. 2 isshown in FIG. 5. There is a sharp pressure discontinuity in the centerof the heat treat assembly 11 due to the lifting plate 14 and base plate16 masses. Larger spacer plate masses increase the pressure differentialbetween the bottom-most and topmost substrates in the heat treatassembly 11. The overall pressure range is small, but has a significantinfluence on the porosity and bulk density of the example porouscarbon-carbon composite. The average pressure is 2.8 psi (0.190 bar)with a range of +/−2.3 psi (0.155 bar) or +/−82% from the top to thebottom, in the example shown in FIG. 2. The density of the substratesheat treated in this configuration varied from 0.47 to 0.56 gm/mL whichis a greater range than is required.

A single top plate 22 is arranged at the top of the stack of tooling andsubstrates to apply additional weight to the heat treat assembly 11. Thetop plate 22 was typically a graphite plate 4 inches (10 cm) thickhaving a diameter similar to that of the base plates 16 and spacerplates 20.

An example heat treat assembly 111 according to this disclosure is shownin FIG. 3. The tooling plates are designed to provide a resulting massthat applies a desired weight to the substrates in a desired location inthe heat treat assembly. The heat treat assembly 111 includes multiplesubstrate assemblies 110, only two of which are shown. A lifting plate14 is arranged at the bottom of the heat treat assembly 111. The liftingplate 14 includes features that enable the heat treat assembly 111 to beinserted and removed in the furnace. In one example, the lifting plate14 has a diameter Dl of approximately 48 inches (122 cm).

Multiple substrates are stacked to form groups 118. In one example,fifty substrates are arranged adjacent to one another and are eachapproximately 0.4 mm thick. The example substrates have a substratediameter d3 of approximately 23 inches (58 cm). Spacer plates 120 arearranged between the substrate groups 118 to apply a weight and preventwarping of the substrates during heat treating. It is desirable tominimize the mass of the spacer plates 120 to reduce the pressurevariation or differential of the substrates from the bottom of the heattreat assembly 111 to the top of the heat treat assembly 111. In oneexample, the spacer plates 120 are ¼ inch thick (0.6 cm), which is thickenough so that the spacer itself does not warp, and include a spacerplate diameter d2 of approximately 23 inches (58 cm).

Typically, lifting plates are arranged between substrate assemblies 110so that fewer than all of the substrate groups 118 can be removed fromthe furnace. The mass of the lifting plates can undesirably increase thepressure variation between the substrate assemblies 110. To this end, itis desirable to minimize the mass of the intermediate lifting plates114. Accordingly, the diameter dl of intermediate lifting plates 114 arereduced. In one example, the intermediate lifting plate 114 isapproximately 35 inches (89 cm) in diameter.

In another embodiment it is desirable to provide an increase mass aboveall of the substrate assemblies 110, which increases the averagepressure in the heat treat stack. One example method of achieving alarger mass at the top of the heat treat assembly 111 using availableheat treat assembly tooling from prior art heat treat assemblyarrangements is shown in FIG. 3. A large lifting plate 14 is arrangedabove the top substrate assembly 110. Multiple base plates 16 arestacked onto the lifting plate 14. Multiple top plates 22 are arrangedabove the lifting plate 14. Additionally, another plate 24 can be addedto the top of the heat treat assembly 111 to increase the mass.

Approaches to obtaining a more uniform pressure distribution within theheat-treat stack includes, for example, minimizing the weight of thetooling between the top most and bottom most substrate. This can beachieved by: maximizing the substrates per spacer, decreasing thethickness of the spacers, decreasing the planform of the spacers so theyare approximately the same size as the substrates, eliminatingintermediate stacking plates, minimizing the thickness and planform sizeof any lifting fixtures and decreasing the height of the stack.

An example number of substrates per spacer is between 25 and 200, with50 to 100 being desirable, for example, to maintain flatness of theheat-treated substrates. The spacer thickness can be between 0.125 inch(0.32 cm) and 0.375 inch (0.0.95 cm), with 0.250 inch (0.64 cm) beingdesirable, for example, as being rigid enough not to deflect duringheat-treating. The spacers are approximately the same size as thesubstrates, and no more than 2 inches (5 cm) larger than the substrates,for example. The intermediate lifting fixture is preferably eliminatedif possible (intermediate lifting fixture 114 present in FIG. 3).

In one example, it is desirable to have a pressure distribution betweenthe topmost and bottom most substrate that is +/−25% to producesubstrates with an acceptable range of bulk densities. This can beachieved with the configuration shown in FIG. 3 that contains 50substrates per spacer. The spacers were 24 inches (60 cm) by 24 inches(60 cm) by 0.25 inch (0.64 cm) thick.

The pressure variation from the top to the bottom of one example stackwas analyzed and is shown in FIG. 6. The average pressure is 3.73 psi(0.267 bar) with a range of +/−0.87 psi (0.066 bar) or +/−23% from thetop to the bottom of the stack compared to an 82% variation in pressurefor the prior art configuration. The predicted density range for the newtooling is 0.51-0.55 g/mL versus 0.47-0.57 g/mL for the prior arttooling which is a more desirable variation. Increasing the number ofsubstrates per spacer from 50 to 100 and eliminating the lifting fixturein the center of the stack for the configuration in FIG. 3 results in afurther reduction in the pressure variation from the top substrate tothe bottom substrate. The average pressure is 3.4 psi (0.23 bar) with arange of +/−0.5 psi (0.034 bar) or +/−14.7% from the top to the bottomof the stack compared to an 82% variation in pressure for the prior artconfiguration.

The average density of the heat-treated substrates is determined by acombination of factors including the density at lamination, theshrinkage that occurs in heat treat which is related to the resincontent and to the weight on top of the heat treating stack. Oneexperienced in the art can systematically vary these parameters toproduce the desired substrate density. The tooling is configured suchthat the pressure variation between the top most part and the bottompart in the heat-treat stack is less than +/−30% and most preferablyless than +/−15%.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of the claims. For that reason, the following claimsshould be studied to determine their true scope and content.

1. A method of heat treating a substrate comprising the steps of:stacking substrates to form a group; determining at least one dimensionfor a plate corresponding to a resulting mass that is greater than apredetermined mass; providing a plate with the at least one dimension;arranging the plate above the group to apply a weight of the plate tothe group; and heating the group; wherein the resulting weightcorresponds to a desired average pressure variation between a bottommost substrate and a top most substrate of the substrate assembly, andthe desired average pressure variation is less than +/− 30%.
 2. Themethod according to claim 1, comprising multiple spacer plates andgroups arranged in alternating relationship to provide a substrateassembly, the substrate assembly arranged on a lifting plate, and a topplate arranged above the substrate assembly, wherein the determiningstep includes determining the at least one dimension for the top plate.3. The method according to claim 1, wherein the desired average pressurevariation is less than approximately +/− 15%.
 4. The method according toclaim 1, wherein the stacking step includes arranging the porouscarbon-carbon composite in groups of between approximately 50 and 200substrates.
 5. The method according to claim 1, wherein the plate is aspacer plate, and the arranging step is arranging the spacer platebetween groups of porous carbon-carbon composites.
 6. The methodaccording to claim 5, wherein the determining step includes determininga weight of the spacer plate.
 7. The method according to claim 5,wherein the determining step includes determining a planform of thespacer plate, the planform being approximately equal to the planform ofthe porous carbon-carbon composite.
 8. The method according to claim 7,wherein the planform of the spacer plate is less than approximately twoinches greater than the planform of the substrate.
 9. The methodaccording to claim 5, wherein the determining step includes determininga thickness of the spacer plate, the thickness approximately between oneeighth to three eighths of an inch.
 10. The method according to claim 9,wherein the height is less than approximately one half inch.
 11. Themethod according to claim 2, wherein a first substrate assembly isarranged on a first lifting plate, a second lifting plate arranged onthe first substrate assembly and a second substrate assembly arranged onthe second lifting plate, wherein the determining step includesdetermining the presence and size for the second lifting plate.